Natural convection on a horizontal smooth cylinder in a cryo-compressed hydrogen tank for trucks

Cryo-compressed hydrogen (CcH2) storage tanks onboard fuel cell electric trucks represent a promising technology for decarbonization of the road transport sector. In order to allow pressure control in the tank, an inner heat exchanger is needed that heats the hydrogen inside the tank and increases the pressure when a minimum pressure level is reached. In this study, the heat transfer from the heat exchanger into the tank is investigated. As the heat exchanger is considered as a horizontal smooth cylinder within the cylindrical tank, the heat transfer is characterized by turbulent natural convection in a concentric-cylindrical annulus with large gap width and large ratio of outer to inner diameter. Further, high temperature difference, increased pressure and cryogenic hydrogen result in a challenging scenario for simulation. Different heat transfer correlations are compared with two-dimensional, steady-state CFD simulation results. Best agreement is achieved with a modified form of the correlation by Kuehn and Goldstein (1978). The mean fluid temperature is defined differently than in the original correlation and is now determined with an energy balance and a separate consideration of fluid properties at the inner and outer walls. The modified correlation shows a mean absolute relative error from the CFD results of 6.9 % with standard deviation of 5.2 % . The calculated convective heat transfer coefficient at the internal heat exchanger surface ranges from α = 96 W/(m 2 K) t o 424 W/(m 2 K) , indicating a high natural convection heat transfer in the CcH2 storage tank. Furthermore, the influence of Boussinesq approximation on the Grashof number is investigated and found to be not significant. • Parametric study of turbulent natural convection in large-gap concentric cylinders. • Focus on high temperature differences in cryo-compressed hydrogen atmospheres. • Steady-state, two-dimensional CFD simulations and comparison of heat transfer results with empirical correlations from literature. • Modification of Kuehn and Goldstein (1978) correlation based on iteratively solved energy balance. • Mean absolute relative error between modified Kuehn and Goldstein (1978) correlation and CFD results of 6.9%